In our prior patent we disclosed methods of removing phosphate from swimming pool water using substantially insoluble particulate lanthanide compounds.
Although the lanthanide compounds are not toxic to algae—and, in fact, non-bio-active in the concentrations employed—we had found that the reduction of pool phosphate concentration to less than about 100 parts per billion (ppb) so nutritionally stressed the algae that normal concentrations of normal pool sanitizers such as chlorine were lethal to algae and provided effective control. This was also disclosed in our prior patent. We also disclosed in our prior patent the method of removing phosphate from pool water in which particulate lanthanide compounds were formed in situ in the pool filter, or were pre-formed and loaded into the pool filter so that phosphate is removed as water is circulated through the filter.
These methods avoided the excessive turbidity or cloudiness that results from mixing lanthanum chloride [solubility about 50%] directly into the pool because of the very fine suspension of precipitated lanthanum phosphate and lanthanum carbonate particles that results. It can take weeks of continuous filtering before the clarity of the pool water is returned to a satisfactory level because many of the lanthanide particles are so fine that they can pass through sand filters. Not only is this level of turbidity and pumping commercially unacceptable, but the lanthanide phosphate particles in the main body of water continue to provide a source of phosphate for algae, albeit at reduced availability, so long as they and the algae are exposed to light. [When retained in the dark within the filter, the lanthanide phosphate is substantially unavailable to algae.]
One method of forming lanthanide particles in the pool filter disclosed in our prior patent was to introduce lanthanum chloride into the filter as an aqueous solution or in particulate form causing lanthanum carbonate and lanthanum phosphate particles to be precipitated onto the filter media by reaction with carbonate and phosphates in water contained in the filter. Circulation of water through the filter then converts the lanthanum carbonate in the filter to lanthanum phosphate, which can be removed by normal backwashing.
We found, however, that the addition of soluble lanthanum salts like lanthanum chloride to the pool filter resulted in lanthanum solution being carried through the filter into the body of pool water so as to cloud the pool. Moreover, where a freshly backwashed sand filter is used, some lanthanum phosphate and carbonate particles that precipitate in the filter can also be carried though the filter media into the pool, adding to pool turbidity. Though the degree of turbidity was less than if the lanthanum chloride had been mixed directly into the bulk of the pool water, it was thought to be undesirable. An alternative method of adding lanthanum chloride to pool water was disclosed in international patent application WO 00/24680 by Zodiac Pool Care, Inc, where a lanthanide halide salt was dispersed in a high molecular weight polymer carrier located in the filter outlet. However, even the slowed release of lanthanum chloride increases pool turbidity.
Our prior patent disclosed methods and compositions that mitigated the above problem by incorporating the finely-divided particulate lanthanide compounds in a carrier that facilitated their retention in a pool filter and even allowed mixing into the bulk of the pool water without significant cloudiness. Products of this type are marketed in Australia and the US by LoChlor under the trademark “Starver”. It is to be noted that loading the Zodiac composition (WO 00/24680) into the pool filter does not eliminate the pool clouding problem, increases the filter back-pressure and reduces the amount of active material that can be introduced into the filter at one time.
International patent application WO 01/62673 (published 30 Aug. 2001) by Natural Chemistry, Inc (licensees of the present applicants) disclosed the use of particulate lanthanum sulphate, which is also substantially insoluble (about 50 grams per liter) in pool water. Lanthanum sulphate particles within the filter are first converted into lanthanide carbonate and then into lanthanum phosphate as water is circulated. However, the calcium sulphate also forms in the filter as the water is circulated. This adds to the volume of particulate material retained in the filter, increases filter pressure and limits the amount of lanthanide that can be added to a given filter at one time. It also appears that calcium from the pool water forms a calcium sulphate coating on the lanthanum sulphate particles in the pool filter thereby reducing their phosphate reactivity. This can be a problem in many areas of the US and Australia where waters are ‘hard’ and have high levels of calcium, where pool water hardness has been incidentally increased by the use of calcium hypochlorite or where pool water hardness has been purposefully increased by the addition of calcium chloride. Moreover, the formation of calcium sulphate is exacerbated by the common use of sodium hydrogen sulphate to decrease the pH of pool water.
Thus, on the one hand, virtually any amount of dissolved phosphate can be removed from pool water by mixing a single dose of lanthanum chloride directly into the pool at the cost of (i) unacceptable pool turbidity, (ii) very slow removal of the precipitated lanthanum phosphate from the water via the filter and (iii) greater risk of excessive filter pressure occurring during the long filtration periods required. But, on the other hand, loading the filter with lanthanide compounds has, hitherto resulted in (i) fine particles being carried through the filter to cause turbidity in the pool, (ii) deactivation of the lanthanide compounds while in the filter, and/or (iii) undesirable limitation of the dose size due to increased filter pressure.